As a technique for mass data transmission, wavelength division multiplexing (WDM) for multiplexing and transmitting a plurality of optical signals having different wavelengths is known (see, e.g., Japanese Laid-Open Patent Publication No. 2012-023607). In addition, for example, orthogonal frequency-division multiplexing (OFDM) for multiplexing a plurality of subcarrier signals where different data are superimposed with high density by means of a digital signal processing (see, e.g., Japanese Laid-Open Patent Publication No. 2012-120010) and a modulation technique for frequency-multiplexing using a Nyquist filter for squaring a spectral waveform (see, e.g., Non-Patent Documents 1 and 2) are also known.
Since an optical signal modulated with a signal having the spectral waveform squared by the OFDM and the Nyquist filter may reduce an occupied bandwidth of the optical signal, an optical transmission system capable of ultra-high density multiplexing with a wavelength-multiplexed optical signal wavelength spacing reduced to a level of a symbol rate has been researched and developed (see, e.g., Non-Patent Document 3).
Related technologies are disclosed in, for example, Japanese Laid-Open Patent Publication Nos. 2012-023607 and 2012-120010.
Related technologies are disclosed in, Tomofumi Oyama et al, “Complexity Reduction of Perturbation-based Nonlinear Compensator by Sub-band Processing,” OFC2015, Th3D (Non-Patent Document 1), Meng Qiu et al, “Subcarrier Multiplexing Using DACs for Fiber Nonlinearity Mitigation in Coherent Optical Communication Systems,” OFC2014, Tu3J (Non-Patent Document 2), and Gabriella Bosco, et al, “On the Performance of Nyquist-WDM Terabit Superchannels Based on PM-BPSK, PM-QPSK, PM-8QAM or PM-16QAM Subcarriers,” JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 29, NO. 1, pp. 53-61, January, 2011 (Non-Patent Document 3).